The number of workers on zero hours contracts has increased by a fifth over the past year, official figures show.

Just over 900,000 people are now employed on the controversial contracts, compared to 747,000 a year ago.

Women make up 55% of those on zero hour contracts, while one in five employed on them is in full-time education.

The Office for National Statistics (ONS) said that these figures show that almost three per cent of the UK workforce is on a zero hours contract.

New TUC analysis has shown that someone employed on a zero hours contract earns on average 50% less than the typical employee. The median hourly wage for zero hours workers is £7.25, while it is £11.05 for others.

TUC general secretary Frances O'Grady slammed the findings: "It is very easy for politicians and employers to talk about the 'flexibility' these contracts offer, but they are not the ones living at the sharp end of the labour market.

"If you don't know how much work you will have from one day to the next, paying the bills and arranging things like childcare can be a nightmare."

A spokesperson for the Business Department hit back, saying: "Since May last year, the use of exclusivity clauses has been unlawful, meaning that individuals have more control over their lives and can work more hours with another employer if they wish.

"Fewer than 3% of the UK workforce classes itself as being on a zero-hours contract in their main job, with almost 70% of those on this type of contract happy with the number of hours they work."

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Vince Cable has said clauses in zero-hour contracts that ban workers from jobs with other employers are "offensive".

Dr Cable told Radio 4's Today programme: "What happens is that people turn up for work under a zero-hours contracts, they’re not guaranteed any work, but they’re banned from working for other people...I think it’s that aspect of it that was offensive and constituted an abuse that we want to stop."

Vince Cable called exclusivity clauses 'offensive'.

The Business Secretary is bringing forward plans to outlaw these 'exclusivity clauses', although he has said that the broader idea of zero-hours contracts can be "perfectly sensible" for some workers.

Tying workers to one business when they are employed on a zero-hour contract is "rigid and archaic", according to a business chief.

Director general of the Institute of Directors (IoD), Simon Walker, came out in support of Vince Cable's move to ban the use of exclusivity clause but allow businesses to continue to use zero-hours contracts.

The IoD has long campaigned against the exploitation of employees on zero hours contracts. Tying an employee into an exclusivity cause can turn a flexible contract into a rigid and archaic one.

The whole point about the flexibility offered by zero hours contracts is that they allow an individual to capitalise on their own time, labour and energy.

We're pleased that the Government recognises the enormous value that flexible contracts can bring to both employer and employee, but at the same time it's right to ensure that exploitation is stamped out.

Vince Cable refused to rule out the use of zero-hours contracts entirely. Credit: PA

Vince Cable announced a clampdown on employers who take on staff with no promise of hours or benefits, and insist they work for no one else.

Despite calls from unions to ban zero-hours contracts outright, Mr Cable said the controversial practice had a place in the labour market - offering working opportunities especially for students and older people.

However, an estimated 125,000 zero-hours workers will benefit from the exclusivity clause ban as they can now look for additional work without fear of losing their current employment, the Business Department said.

A recent report by the Office for National Statistics estimated that employers held 1.4 million contracts that did not guarantee a minimum number of hours.

Lack of clear information and exclusivity clauses in zero hours contracts can leave employees "feeling vulnerable", the Business Secretary has said.

Vince Cable said Acas' research supported what the Government had found in their review of zero hours contracts:

Whilst zero hours contracts work for some, the use of exclusivity clauses and the lack of clear information can leave employees feeling vulnerable.

That is why I launched a consultation last autumn which looked into the issues of transparency and exclusivity in the use of zero hours contracts, to ensure people are getting a fair deal.

I want to make sure those looking to work flexibly under these types of contracts understand their rights and are not prevented from topping up their income by being tied exclusively to one employer. We will publish our response to the consultation shortly.

Workers on zero hours are too afraid to look for work elsewhere because they already feel committed to one employer, the survey showed. Credit: PA

Conciliation service Acas said it was receiving around 70 calls a week about zero hours contacts, and a feeling of "effective exclusivity" of being tied to a single employer was emerging as a major concern.